Dense wavelength division multiplexing systems (DWDM), for example, used in the optical communication field require optical attenuation and an optical shutter function for the adjustment of the level of signal light amplified by erbium-doped fiber amplifiers (EDFA). A variable optical attenuator is used for such purposes.
Various types of variable optical attenuators have been developed. An opposed-type attenuator is one typical example, and this includes a polarizer and an analyzer, which are wedge-shaped birefringent plates, arranged at both sides (the input side and the output side) of a Faraday rotational angle varying unit on its optical axis. The Faraday rotational angle varying unit that is incorporated generally applies external magnetic fields from at least two directions to a Faraday element (for example, a magnetic garnet single crystal film having Faraday effect) and changes a composite magnetic field obtained therefrom to control the Faraday rotational angle of the light beam passing through the Faraday element.
Incident light from an input fiber is collimated into collimated light by a lens, passes through the polarizer, the Faraday element of the Faraday rotational angle varying unit, and the analyzer in this order, is condensed by a lens, and is then introduced into an output fiber. A permanent magnet applies a stationary magnetic field to the Faraday element in the direction parallel to the optical axis, and an electromagnet applies a variable magnetic field that is perpendicular to the optical axis. When the direction of magnetization of the Faraday element is changed by changing the composite magnetic field, the Faraday rotational angle changes, and thereby the amount of light passing through the analyzer can be controlled.
Such a variable optical attenuator, which employs the wedge-shaped birefringent plates as the polarizer and the analyzer, is of the polarization non-dependence type, and therefore, it is particularly suitable for a fiber coupling type device. In order to achieve a high-attenuation shutter function with this structure, it would only be necessary to connect the variable optical attenuators of the above-mentioned type in a two-stage cascade. With such a configuration, however, the number of components becomes large, and a problem involving increase in size arises.
In recent years, a reflection-type structure has been proposed for a variable optical attenuator (refer to, for example, JP-A 10-161076). This attenuator includes a two-core ferrule provided with an input fiber and an output fiber, a lens, a wedge-shaped birefringent plate, a magnetooptical crystal (Faraday element), and a reflecting mirror, and is configured such that a magnetic field is applied to the magnetooptical crystal to change the Faraday rotational angle. This reflection-type variable optical attenuator can be formed in a small size and can be made to be polarization non-dependent by using the wedge-shaped birefringent plate.
In this reflection-type variable optical attenuator with such a structure, however, there exists an angle, with respect to the reflecting mirror, between the input light and the output light (the reflected light) which is dependent on the focal distance of the lens, and thus, the pitch between the input and the output fibers of the two-core ferrule is predetermined. Therefore, there are design limitations to the distance between the end of the ferrule and the reflecting mirror. Further, since the light from the wedge-shaped birefringent plate is emitted at an angle, the light has to be incident obliquely on the plate. Therefore, the reflection-type attenuator is sensitive to incident angle and is difficult to manufacture. Furthermore, the reflection-type attenuator has a problem that it is incapable of achieving a high-attenuation shutter function.
It is an object of the present invention to provide a variable optical attenuator that is capable of achieving a high-attenuation shutter function and that is easy to manufacture. Another object of the present invention is to provide a variable optical attenuator that can be made smaller compared to a conventional two-stage cascade connection type attenuator and that can achieve power saving and reduction in the number of components while being provided with a high-attenuation shutter function. A further object of the present invention is to provide a variable optical attenuator having a structure that allows to halve the space necessary for mounting the input and output fibers, that allows installation even in corners, and that has an increased degree of freedom of selecting a position for installation while being provided with a high-attenuation shutter function. A further object of the present invention is to provide a reflection-type variable optical attenuator in which it is possible to reduce wavelength-dependent losses and also reduce temperature-dependent losses.